如何使用Spring加载属性文件？

@Configuration
@PropertySource("classpath:config/config.properties")
public class AppConfig {
    // ...
}

@Value("${app.name}")
private String appName;

@Autowired
private Environment env;

public void printAppVersion() {
    String appVersion = env.getProperty("app.version");
    System.out.println("App version: " + appVersion);
}

@Configuration
public class AppConfig {
    @Bean
    public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
        PropertySourcesPlaceholderConfigurer configurer = new PropertySourcesPlaceholderConfigurer();
        configurer.setLocation(new ClassPathResource("config/config.properties"));
        return configurer;
    }
    
    @Value("${app.name}")
    private String appName;
    
    // ...
}